Polyolefins are widely used in demanding polymer applications wherein the polymers must meet high mechanical and/or electrical requirements. For instance in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications the electrical properties of the polymer composition has a significant importance. Furthermore, the electrical properties of importance may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.
A typical power cable comprises a conductor surrounded, at least, by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. Power cable is defined to be a cable transferring energy operating at any voltage level. The voltage applied to the power cable can be alternating (AC), direct (DC) or transient (impulse). Moreover, power cables are typically indicated according to their level of operating voltage, e.g. a low voltage (LV), a medium voltage (MV), a high voltage (HV) or an extra high voltage (EHV) power cable, which terms are well known. EHV power cable operates at voltages which are even higher than typically used for HV power cable applications. LV power cable and in some embodiment medium voltage (MV) power cables usually comprise an electric conductor which is coated with an insulation layer. Typically MV and HV power cables comprise a conductor surrounded at least by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order.
Electrical Conductivity
In DC power cables the DC electrical conductivity is an important material property e.g. for insulating materials for high voltage direct current (HV DC) cables. First of all, the strong temperature and electric field dependence of this property will influence the electric field. The second issue is the fact that heat will be generated inside the insulation by the electric leakage current flowing between the inner and outer semiconductive layers. This leakage current depends on the electric field and the electrical conductivity of the insulation. High conductivity of the insulating material can even lead to thermal runaway under high stress/high temperature conditions. The conductivity must therefore be sufficiently low to avoid thermal runaway.
Accordingly, in HV DC cables, the insulation is heated by the leakage current. For a specific cable design the heating is proportional to the insulation conductivity×(electrical field)2. Thus, if the voltage is increased, far more heat will be generated.
JP2018811A discloses an insulation layer for a DC cable which contains a blend of 2-20 wt % of a high density polyethylene with a low density polyethylene. It is stated that blend provides improved DC breakdown and an impulse property. The blend is mixed with 2-3 wt % of a crosslinking agent. The type and layer structure of the cable has not been specified.
WO0137289 discloses a specific thermoplastic propylene homo- or copolymer for a cable layer material in LV, MV and HV AC cables, as well as in telecommunication cables. The applicability of the material to DC applications is not discussed.
There are high demands to increase the voltage of a power cable, and particularly of a direct current (DC) power cable, and thus a continuous need to find alternative polymer compositions with reduced conductivity. Such polymer compositions should preferably also have good mechanical properties required for demanding power cable embodiments.
The invention and further objects and benefits thereof are described and defined in details below.